Individual trees directly modify the soils they occupy, which in turn can affect the survival and growth of tree seedlings–a process known as plant-soil feedbacks (PSFs). PSFs critically influence tree species composition and diversity in forests. Despite their importance, the persistence of PSFs after tree death remains virtually unstudied. To examine PSFs at a range of times since tree death, we constructed a chronosequence of harvest times (0.5, 1.5, 3.5, & ~15 years since harvest) of Prunus serotina, which previously has shown strong negative PSFs. At each harvest site, we collected soil cores from the vicinity of P. serotina stumps and live control trees. To account for the possibility that increased numbers of conspecific neighboring adults may strengthen PSFs, we also recorded the number of living conspecific neighbors within 20 meters of each stump and live tree. We also collected soil cores from a stand with very low density (1 tree/ha) of P. serotina as an additional control. In the greenhouse, we assessed PSF persistence by growing a conspecific seedling in each soil core under two light levels (5% & 30% full sun) and monitored survival for 12 weeks.
Results/Conclusions
Negative PSFs persisted after tree removal, but for less than 1 year and only under low light conditions. Thus, seedlings were released from negative PSFs a year or more after tree death, suggesting that rapid belowground turnover occurs when a canopy tree is lost despite the potential for fungal components of PSFs to persist long term as spores. Seedling survival decreased with increasing numbers of conspecific live trees when a live focal tree was present or when the tree had been recently harvested (within 0.5 yrs.). This indicates that conspecific neighbors can amplify the effects of PSFs on seedlings and aligns with expectations of the Janzen-Connell hypothesis. Lastly, where P. serotina’s density was exceptionally low in the area, all seedlings survived the duration of the study demonstrating the absence of any soil-borne organisms in sufficient numbers to cause P. serotina seedling mortality. Overall, these results also indicate that PSFs could be operating at multiple spatial scales simultaneously – the 1 ha community level, the 20 m local neighborhood, and at the single mature tree scale – to influence seedling survival and forest dynamics.